1. Field of the Invention
The present invention relates to storage area networks, and more particularly to transferring data between storage devices using a Fibre Channel switch.
2. Background of the Invention
Storage area networks (“SANs”) are commonly used where plural memory storage devices are made available to various host computing systems. Data in a SAN is typically moved from plural host systems (that include computer systems, servers etc.) to a storage system through various controllers/adapters.
Host systems often communicate with storage systems via a host bus adapter (“HBA”, may also be referred to as a “controller” and/or “adapter”) using an interface, for example, the “PCI” bus interface.
FIG. 1A shows a block diagram with a host system 10 having a HBA 11 coupled to a Fibre Channel switch 12. Switch 12 is also coupled to storage system 14 and 20. Storage system 14 includes HBA 13 and is coupled to storage devices 15, 16 and 17. Storage system 20 with HBA 21 is coupled to storage devices 18 and 19. The term storage device in this context includes, disk, tape drives or any other media used for storing electronic information.
Host system 10 typically includes several functional components. These components may include a central processing unit (CPU), main memory, input/output (“I/O”) devices (not shown), read only memory, and streaming storage devices (for example, tape drives).
Storage devices (for example, 15, 16, 17, 18 and 19) are coupled using the Small Computer Systems Interface (“SCSI”) protocol and use the SCSI Fibre Channel Protocol (“SCSI FCP”) to communicate with other devices/systems. Both the SCSI and SCSI FCP standard protocols are incorporated herein by reference in their entirety. SCSI FCP is a mapping protocol for applying SCSI command set to Fibre Channel.
Fibre Channel is a set of American National Standard Institute (ANSI) standards, which provide a serial transmission protocol for storage and network protocols such as HIPPI, SCSI, IP, ATM and others. Fibre Channel provides an input/output interface to meet the requirements of both channel and network users.
Fibre Channel supports three different topologies: point-to-point, arbitrated loop and Fibre Channel fabric. The point-to-point topology attaches two devices directly. The arbitrated loop topology attaches devices in a loop. The Fibre Channel fabric topology attaches host systems directly to a fabric, which are then connected to multiple devices. The Fibre Channel fabric topology allows several media types to be interconnected.
In Fibre Channel, a path is established between two nodes where the path's primary task is to transport data from one point to another at high speed with low latency, performing only simple error detection in hardware.
Fibre Channel fabric devices include a node port or “N_Port” that manages fabric connections. The N_port establishes a connection to a fabric element (e.g., a switch) having a fabric port or “F_port”. Fabric elements include the intelligence to handle routing, error detection, recovery, and similar management functions.
A Fibre Channel switch (for example, 12) is a multi-port device where each port manages a simple point-to-point connection between itself and its attached system. Each port can be attached to a server, peripheral, I/O subsystem, bridge, hub, router, or even another switch. A switch receives messages from one port and automatically routes it to another port. Multiple calls or data transfers happen concurrently through the multi-port Fibre Channel switch.
Fibre Channel switches use memory buffers to hold frames received and sent across a network. Associated with these buffers are credits, which are the number of frames that a buffer can hold per fabric port.
Fibre Channel storage devices using the SCSI FCP protocol typically use the client/server model. Typically, the client is a host system with an HBA (an “Initiator”) such as a file server that issues a read or write command to a “Target”. The Target may be a disk array that responds to the client request. Most storage devices such as disk drives or tape drives are SCSI target devices. Initiator devices (usually host bus adapters on server computers) start all IO operations.
In a storage area network, backing up data, or any operation where large amounts of data is sent from one storage device to another, usually involves a server (host 10) reading data from one storage device (for example, disk 18) to the server, then writing it to the destination storage device (for example, disk 15). This operation is slow and inefficient.
Therefore, there is a need for a system for efficiently transferring data between two storage devices.